Silicone rubber can be made electrically conductive by using conductive carbon black as the filler. Acetylene blacks are found to be very useful for this purpose. Conductive silicone rubber gives the typical benefits of heat resistance, weather resistance, low temperature flexibility, and the like, of ordinary silicone rubber. Such rubber is finding increasing usefulness as an elastic heating medium (heat results from current flow), as well as in any electrical application where low electrical resistance is required. In general, a broad range of conductivities may be obtained in silicone rubber in incorporating varying amounts of the conductive carbon black into the formulation. The higher the carbon black content of the compound, the greater will be the conductivity, or lower will be the volume resistivity. In the present state of the art, the most desirable conductive compositions will exhibit a volume resistivity of not more than about 10,000 ohm-cm., a Shore A hardness of at least 50, a tensile strength of greater than 450 pounds per square inch and elongation of greater than 100 percent at break.
The specific requirements needed by conductive silicone rubber make it difficult to extend conventional silicone elastomer technology to this field of use. For example, the material must contain a type of carbon black that will conduct electricity, but all conductive blacks so far investigated can be cured only with peroxides, using compression molding or steam vulcanization, both of which require expensive equipment and high energy requirements. The catalyst which is normally used to vulcanize silicone elastomers with hot air, which is a cheaper, less energy and equipment intensive means of curing, is 2,4-dichlorobenzoyl peroxide. Unfortunately, however, silicone rubber compositions containing conductive blacks react with and deplete the 2,4-dichlorobenzoyl peroxide catalyst, and hot air vulcanization does not occur.
Many approaches to overcoming these problems and providing a hot air vulcanizing composition containing conductive blacks have been tried, but without success. These approaches have included substituting cross-linking agents, various other conductive blacks and catalyst compositions, and the substitution of platinum addition catalysts for the peroxide.
It has now been surprisingly discovered that if a peroxide catalyst different from the preferred prior art hot air vulcanization catalyst is substituted, there is obtained a composition which cures with compression molding or steam, but not with hot air. Moreover, if this composition is then treated with a small amount of platinum addition catalyst, not sufficient to cause cross-linking within about 24 hours at room temperature, 23.degree. C., there will be obtained a useful conductive compound which will permit the materials to be mixed with a reasonable shelf age and still provide an extrudable product which, surprisingly, hot air cures. If too much of the platinum addition catalyst is used, such as a conventional amount, the rubber composition will tend to cross-link and become unusable in a short period of time, e.g., of the order of 2-4 hours. Using the platinum catalyst alone at a lower concentration extends the shelf age, but only provides a semi-cured rubber after hot air vulcanization. The foregoing indicates the need for using a combination of peroxide and platinum catalyst in a relatively narrow use range.